1. Field of the Invention
The present invention relates to a hologram recording and reproducing apparatus for recording a hologram in a hologram recording medium and reproducing the information recorded in the hologram.
2. Description of the Related Art
A conventional hologram recording and reproducing apparatus is disclosed in JP-A-2002-216359, for example. The hologram recording and reproducing apparatus is basically configured as shown in FIG. 6(a). Specifically, a laser beam emitted by a laser source (not shown) is split into a recording beam S and a reference beam R, out of which the recording beam S is modulated by a spatial light modulator 500 into a light representing two-dimensional information and then emitted onto a recording layer 92 of a hologram recording medium B at a predetermined incident angle, through an objective lens 700. Meanwhile, the reference beam R is directed to the hologram recording medium B by a galvanomirror 900 and condenser lenses 100A, 100B, so as to intersect with the recording beam S on the recording layer 92 of the hologram recording medium B. The incident angle of the reference beam R with respect to the hologram recording medium B can be adjusted at different angles via an operation of the galvanomirror 900, as shown in solid lines and broken lines in FIG. 6(a). As a result, the recording beam S and the reference beam R create interference fringes that constitute a hologram H on the recording layer 92 of the hologram recording medium B, and different patterns of interference fringes are multi-recorded in the hologram H, according to the incident angle of the reference beam R.
For reproduction of the information recorded in the hologram H, the reference beam R is emitted onto the hologram recording medium B at the same incident angle as that at the time of recording, and the light returning from the hologram recording medium B (herein after, reflected beam) is received by a photodetector (not shown). Such reflected beam is emitted from the hologram recording medium B at an output angle that is the same as the incident angle of the recording beam S, thus reproducing the recording beam S. Accordingly, based on a detected signal by the photodetector which has received the reflected beam, the two-dimensional information (recorded information) contained in the recording beam S can be reproduced.
For better understanding on the reproduction principle of the hologram H, description on wave number vector space will be given referring to FIG. 6(b). The wave number vector space is defined as a unit circle K having a radius represented by a wave number (2π/λ), which is the inverse number of a value obtained via dividing the wavelength λ of the emitted laser beam by 2π. When the recording beam S is designated by a wave number vector Ks and the reference beam R corresponding to a certain incident angle is designated by a wave number vector Kr, the directions of the recording beam S and the reference beam R correspond to the directions of the wave number vectors Ks, Kr, and the respective initial points of the wave number vectors Ks, Kr are placed at the center of the unit circle K. Since the recording beam S and the reference beam R are split from the laser beam emitted by the identical light source, the magnitudes of the wave number vectors Ks, Kr are identical, and the respective terminal points of the wave number vectors Ks, Kr fall on the circumference of the unit circle K. When the hologram H created by the recording beam S and the reference beam R is designated by a wave number vector Kh, the wave number vector Kh of the hologram H can be defined as a composite vector of the wave number vector Ks of the recording beam S and an inverted vector of the wave number vector Kr of the reference beam R, and the initial point and the terminal point of the wave number vector Kh both fall on the circumference of the unit circle K. Accordingly, unless the wave number vector of the hologram does not fit the unit circle, the hologram is not recorded.
Also, the wave number vector of the reflected beam at the time of reproduction can be defined as a composite vector (not shown) of the wave number vector Kr of the reference beam R and the wave number vector Kh of the hologram H. Based on this, when the wave number vector Kr of the reference beam R at the time of reproduction is the identical vector as that at the time of recording, the wave number vector of the reflected beam results the same as the wave number vector Ks of the recording beam S. Consequently, unless the wave number vector of the reflected beam does not fit the same unit circle as that of the wave number vector of the reference beam at the time of reproduction, the hologram cannot be reproduced.
In the conventional hologram recording and reproducing apparatus, however, the intensity of the laser beam emitted by the light source may fluctuate between at the time of recording and at the time of reproduction. Such fluctuation in the laser beam intensity may lead to a difference in wavelength between at the time of recording and at the time of reproduction, thus inhibiting the detection of the reflected beam at the time of reproduction.
To be more detailed, when the wavelength at the time of reproduction is longer than that at the time of recording, a unit circle K′ of the wave number vector space and a wave number vector Kr′ of the reference beam R are formed, as indicated by broken lines in FIG. 7(a). The unit circle K′ at the time of reproduction becomes smaller than the unit circle K at the time of recording. Accordingly, even though the wave number vector Kr′ of the reference beam R at the time of reproduction is oriented in the same direction as that at the time of recording, the wave number vector of the reflected beam does not fit the unit circle K′ corresponding to the wave number vector Kr′, and therefore, based on the principle, the reflected beam cannot be obtained.
Likewise, when the wavelength at the time of reproduction is shorter than that at the time of recording, a unit circle K″ of the wave number vector space and a wave number vector Kr″ of the reference beam R are formed, as indicated by broken lines in FIG. 7(b). The unit circle K″ at the time of reproduction becomes larger than the unit circle K at the time of recording. Accordingly, even though the wave number vector Kr″ of the reference beam R at the time of reproduction is oriented in the same direction as that at the time of recording, the wave number vector of the reflected beam does not fit the unit circle K″ corresponding to the wave number vector Kr″, and therefore, based on the principle, the reflected beam cannot be obtained.